crypto/internal/nistec: use Booth multiplication in purego P-256
Brings ScalarMult from 71 adds/op + 259 doubles/op to 58 adds/op + 263
doubles/op and ScalarBaseMult from 64 adds/op to 42 adds/op, matching
the assembly scalar multiplication algorithm.
Change-Id: I6603b52d1c3b2c25ace471bd36044149f6e8cfab
Reviewed-on: https://go-review.googlesource.com/c/go/+/627937 Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>